University of Tehran 1

Interface DesignKeyboard and Printer

Omid Fatemi

University of Tehran 2

Typical Interface Design

Connect Compute Convey Cooperate

Sense RealityTouch RealityConnectTransform

Embedded SystemsMicrosAssembler, CReal-TimeMemoryPeripheralsTimersDMA

PC interfacesHCI

BussesProtocolsStandardsPCIIEEE488SCSIUSB & FireWireCAN

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Outline

• Keyboard design

• Key detection

• Keyboard buffer

• Centronics printer interface

• IO printer ports

• Parallel port types

• Interfacing LPT ports to various devices

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Keyboard Layout

• Linear layout– N input ports for N keys

• Matrix layout– 2N ports (N input and N output) for N^2 keys

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Scanning and Identifying

• Micro controller

• Rows to output ports

• Columns to input ports

• Grounding rows

• Reading the columns

• Grounding one row

• Reading the columns

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Detection Algorithm

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Assembly Code

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PC Keyboard

• Micro controller (8042)– Microprocessor, RAM, EPROM, I/O ports

• Detection by 8042• Scan code assign• Serial communication to motherboard

– One start bit (0)– 8 bits for scan code– Odd-parity bit– One stop bit (1)

• 74LS322 (serial-in-parallel-out SR) in motherboard

• 74322 to port A of 8255 (port 60H)• In AT and later PCs another 8042 in motherboard

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PC Scan Codes

Make (0-7FH and break (plus 80H)

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Advanced Scan Codes

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Status Bytes

0040:0017 H (first) 0040:0018 H (second)

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Printer

Printer and Printer Interfacing

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Centronics printer interface

• 36-pin

• Grounds to reduce electrical noise (signal has

its own ground return)

• Data lines

• Printer status signals

• Printer control signals

• Ground signals

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Pin Layout

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Printer Connector

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Printer-PC Communication

• PC checks busy

• If not busy, PC puts 8-bit data

• PC activates #STROBE (.5 micro)

• Data must stay (.5 micro after STROBE)

• Printer assert busy

• After reading data printer asserts #ACKNLG (5 micro)

• De-assert busy

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I/O Base Address for LPT

• -d 0040:0008

• LPT: 03BC, 0378, 0278

• 3 I/O ports– – LPT data lines

– – LPT status lines

– – LPT control lines

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Printer’s Ports

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BIOS: INT 17H

• AH=0– Print a character– AL: ASCII– DX: LPT number– AH returns status

• AH=1– Initialize

• AH=2– Get the status

• Printer time out?– 20 seconds (-d 0040:0078)

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Special Characters

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Inner Working of INT 17

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Printer Timing

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Buffering Ports

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Using Parallel Ports

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Stepper Motor

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Selected Problems

• 5, 6, 8

• 22, 28, 32, 43

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Summary